Focus on the heart of the HVAC system

Sales Director of Applied Business at SKM Air Conditioning Asim A. Hashmi on the critical nature of chiller selection and performance.

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MEP and HVAC, SPECIAL REPORTS, SKM, Air conditioners, Chillers, Asim A. Hashmi

How do you select the best air-cooled chiller for your project? The chiller is the heart of the HVAC system and the owner will spend 60-70% of his building utility bills on electricity through it.

At the same time, chiller performance is critical in meeting indoor temperature requirements.

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Air-cooled chiller selection depends on project location, maximum ambient temperature, building cooling load profile, total building load, number of chillers required, chilled water system design, availability of power, and plant room space.

Major Cities in the GCC are located near the sea, resulting in high corrosive conditions due to temperature and humidity. Dust and sandstorms make it worse. 

It is recommended to split the building cooling across a minimum of two chillers, based on total cooling load and diversity. Air-cooled screw chillers’ range for most manufacturers is from 75TR to 500TR (nominal capacity at 35oC).

Spacing between the chillers and free air flow on the condenser play a major role on performance and electricity consumption. 

With advancement in control systems, chillers can now work with Primary Variable Pump Systems, saving chilled water pumping power and installation cost.

Chiller should be able to operate till 52oC without compromising its performance (unloading the compressors) and shouldn’t trip up to 55oC – chillers are supposed to work when you need it the most.

HVAC system designer plays a major role in identifying the right specifications of a chiller, based on the above requirements.

Following are key points in designing and selecting chillers in the Gulf:

Air Cooled Condenser
Meeting high ambient requirements with high EER or COP is challenging.

To overcome this task, the condenser area should be large enough to reject complete compressors heat.

In case of a limited condenser area, a compressor will be unable to handle the lift.

Chiller safety will unload the compressor and, therefore, the building will not get the desired cooling load at peak temperature.

Sandstorms are a common occurrence of the region and therefore condenser coils are to be constructed with seamless copper tubes and mechanically expanded into plated aluminium fins.

Chillers should have an economizer section between the condenser and the main expansion valve to increase the refrigerant sub-cooling before refrigerant enters the evaporator.

Achieving 100% liquid refrigerant is crucial for the expansion valve to perform to its best. 

Slit fins, also called rippled or slotted or louvred fins, are not recommended because they will clog dust on the condenser coils, reducing chiller capacity and efficiency during operation.

It may also cause corrosion during high humid conditions.

The international market has switched to micro-channel condensers for chillers due price competitiveness.

But there is a big question mark for its suitability in the sandy and humid climate of the Gulf. In my opinion, copper tubes and aluminium plate fins for condensers are the best option.

Condenser Coil Coating
Condenser coils exposed to the industrial environment should be protected against corrosion with dipped and baked coating. Pre-coated fins or spray coating on site cannot cover the complete coil.

Compressor
The majority of the buildings with chilled water system design will fall above a 150TR building load. Hence the natural choice will be high efficiency screw technology compressor chillers.

Screw chillers (minimum two circuits) have the ability to vary the cooling output capacity from 100% to 10% via the use of a slide valve to limit refrigerant delivery to the compressor and provide a smooth, modulating transition between capacities.

Evaporator
Shell and Tube evaporators with latest technology straight tube Dry Expansion type or Flooded type should be used.

Coolers should be suitable for Variable Primary Flow as it provides better system efficiency.

Starter & Controls
Chiller starter and control components should be designed to withstand a high temperature control panel.

Reliability of the chiller depends on the microprocessor controller operation because, in case of its failure, the whole chiller will shut down. IP 54 rating is the minimum requirement of control panel. You can have Star-Delta, Solid State or VFD Starter.  

Performance and reliability of a chiller depends on the selection of the components and its specifications.

You can have a very efficient chiller for T1 (35oC) condition, but it may not perform and withstand in (46-52oC) – the harsh climatic conditions of the Gulf.

Therefore, it needs to be designed and selected accordingly.

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